block/m25p80: Update Micron entries

- Split 32Mb and 256Mb parts into a11 and a13 variants.
- Add the 4K sector flag to the 128Mb parts.  (These entries were taken from
  the Linux kernel list, which is missing the flag.)
- Fill out the table of sizes with entries for 64Mb parts.
configure: Generate config-host.ld only if used

Also use ${config_host_ld} variable instead of expanding its content in
one spot.
configure: Disable host-bsd USB on FreeBSD

It hasn't built since FreeBSD 8.x, and is disabled by a patch in the
FreeBSD ports tree.  FreeBSD is migrating to QEMU's libusb support.